Enhanced vision systems (EVS) increase vehicle operators' situational awareness in low-visibility conditions. Radar, for example, helps a ship captain maneuver in fog. Forward-looking infrared (FLIR) sensors can help a helicopter pilot see animals in a landing zone at night. EVS are based on one or more sensor technologies such as radar, FLIR, lidar, or video cameras sensitive to various wavelengths or spectral bands. Each of these sensors can see through some visibility impairments, but not others. A certain type of radar may be able to see through fog, but it may not be able to see through dust, for example. Hence the meaning of “clear” or “obstructed” visibility depends on the specific type of EVS sensor. Fog may be clear to some EVS sensors, but obstruct others.
Images from an EVS may be displayed on primary flight display or multifunction display in a glass cockpit or may be shown on a transparent, head-up display (HUD) (or helmet-mounted display (HMD) or head-worn display (HWD)) that is placed in the pilot's through-the-window field of vision. HUDs, HMDs and HWDs are especially compelling as they superimpose enhanced images on a natural view.
EVS images may become degraded for various reasons, however. Environmental factors such as dust, fog, smoke, clouds and snow may obstruct visibility. Helicopter brownout is a kind of obscuration caused by dust kicked up by a helicopter's downwash, for example. Brownout can quickly change a routine landing into a potentially deadly situation for helicopter pilots if they lose situational awareness near the ground. Electromagnetic interference or deliberate jamming may also degrade image quality.
FIGS. 1A and 1B illustrate a helicopter approaching a landing zone and suffering from brownout. In FIG. 1A helicopter 100 approaches landing zone 105 in clear conditions. Just prior to touchdown, the view of landing zone 105 is obscured by dust cloud 110 as shown in FIG. 1B. The dust cloud not only obscures the pilot's natural vision, but also degrades images from EVS. The degraded EVS images then become distractions to the pilot. The transition from the situation of FIG. 1A to that of FIG. 1B can take just a few seconds.
FIGS. 2A and 2B show an example of EVS images as image degradation due to brownout occurs. FIG. 2A shows clear conditions (i.e. clear to the particular type of sensor in use) while FIG. 2B shows the same scene when the sensor's view is obstructed by dust. Sometimes images from one type of sensor will be clear, i.e. like FIG. 2A, while images from another type of sensor are obstructed, i.e. like FIG. 2B, at the same time. Snow, fog and smoke may also obstruct sensor visibility.
Degraded EVS images are an unwanted, and possibly harmful, distraction to pilots. Degraded images may even obscure a pilot's view when presented on a HUD. Conventional EVS include a disable switch on the control yoke in an aircraft. This allows pilots to quickly turn off degraded EVS images, but it also means that pilots may not take full advantage of EVS. Pilots may leave EVS disabled in situations where it would actually be beneficial because they do not want to deal with potential distractions. Pilots may also leave EVS disabled if they are unaware that useful images have become available again after a period of image degradation.
Methods have been proposed to indirectly detect when an image might be degraded. Helicopter brownout conditions can be predicted to a certain extent based on altitude and airspeed, for example. Other indirect methods employ sensors to try to detect sources of obscuration such as smoke, rain or jamming. However, none of the existing methods directly detect whether or not an image is actually degraded.
What are needed are systems and methods that automatically and reliably monitor the quality of EVS images so pilots can realize EVS' full benefits.